Remove support for attribute event listeners.

client/dom/generated/src/frog/EventSource.dart

 
 class EventSource native "EventSource" {
 
   String URL;
 
-  EventListener onerror;
-
-  EventListener onmessage;
-
-  EventListener onopen;
-
   int readyState;
 
   void addEventListener(String type, EventListener listener, [bool useCapture = null]) native;
 
   void close() native;
 
   bool dispatchEvent(Event evt) native;
 
   void removeEventListener(String type, EventListener listener, [bool useCapture = null]) native;
 
   var dartObjectLocalStorage;
 
   String get typeName() native;
 }